With the development of world-wide metallurgy technology and advances of modern continuous casting technology, castable steel is continuously expanded, and some steel with high alloy, high quality and high crack sensitivity has been continuously produced by continuous casting process in large steel enterprise. However, in the production of plates and strips, a sliver or peeling-off defect at the edges of the hot rolled plates and strips is a common problem puzzling modern metallurgical workers all the time.
Particularly, with the recent development of continuous casting and rolling technology, energy is greatly saved caused by the slab hot charge directly to heating furnace. So, the continuous casting and rolling technology is widely applied to the production in metallurgy enterprises. However, the adverse effect brought by the continuous casting and rolling technology is all surface defects (including corner transverse cracks, longitudinal cracks and the like) of the casting slab are inevitably directly reflected on the surface of a final rolled strip because the casting slab can not be cleaned off line. It leads to the development and application of a chamfered mold technology.
By adopting the chamfered mold technology, each original right angle of the casting slab becomes two obtuse angles, to eliminate stress concentration in the bending and straightening process, so as to radically eliminate corner transverse cracks of the continuous casting slab. However, because the shape of the slab corners produced by a chamfered mold is limited, e.g. the angle of the chamfered casting slab is generally 25°-45° and the length of the chamfered surface is 30-85 mm, larger chamfer angle (e.g. more than 45°) of the casting slab produced by adopting the chamfered mold would bring two defects: 1) the service life of the chamfered mold is greatly shortened; and 2) the risk of breakout caused by slab off corner longitudinal cracks is increased. Therefore, the angle of the chamfered mold is less than 45° in the prior art.
Although the problem of transverse corner cracks of the casting slab may be effectively solved by adopting the chamfered mold technology, many production practice results show that edge slivers on plates and strips can not be completely eliminated by adopting the chamfered mold technology with above-mentioned parameters.
Thus, some foreign metallurgical researchers developed some methods for rolling casting slab corners, to obtain more reasonable slab corner shapes.
For example, Japanese patent No. P2001-18040A titled ‘Production Method of Continuous Casting Slab’ providing a device equipped with squeezing and rolling back-up rolls at four corners of a casting slab right below the mold, wherein a rectangular casting slab comes out from the mold and having four corners is squeezed and rolled into a casting slab with eight corners. Thus, corner defects of the casting slab in the following rolling process are avoided. In practical production, the casting slab which has a relatively thin slab shell and filled with molten steel is rolled at the outlet of the mold, so that the corners of the casting slab are deformed, each right angle is rolled to be flat and becomes two obtuse angles larger than 90°, and this is completely infeasible and also very dangerous.
To avoid a breakout risk caused by rolling the corners of the slab shell below the outlet of the mold, in Japanese patent No. S63-215352 titled ‘Continuous Casting device’, rolls for rolling corners of a casting slab are arranged on straightening machine in caster roll arrangement. Although this method may avoid the breakout risk caused by rolling the corners of the casting slab, in a modern slab continuous casting machine, the straightening machine is arranged into caster segment, and rolls for rolling the four corners of the casting slab at large rolling reduction and a driving device may not be arranged at all in the compact structural design. In addition, because the transverse corner cracks on slab is inevitable, the slab transverse corner cracks become defects on the surface of a rolled product by adopting such rolling method, and the defect only can be cleaned by trimming. Therefore, the yield is greatly reduced.
In Japanese patent No. H6-320204 titled ‘Rolling Mill for Chamfering and Method for Chamfering Corners of Continuous Casting Slab’, a dedicated rolling mill is arranged at the rear part of a continuous casting machine which roll a rectangular continuous casting slab, so that the corners of the rectangular continuous casting slab become large chamfers, to eliminate the slivers on a plate in the following rolling process. This method has two shortcomings: 1) the equipment investment is expensive because the slab corners need large deformation, and 2) as mentioned above, the slab transverse corner cracks are inevitable, and the slab transverse corner cracks will become defects on the surface of the rolled product by adopting such rolling method, so that the yield is greatly reduced.